Using a primary mammary epithelial cell culture system, we found that epidermal growth factor (EGF), a peptide hormone synthesized and secreted by the salivary gland, stimulated mammary cell proliferation and inhibited mammary cell differentiation. These effects of EGF were apparent at the physiological concentrations. The mRNA sequence of whey acidic protein, a major mouse milk protein, was cloned in full length. The sequence analysis of the cDNA clones revealed that the mRNA contains a 28 nucleotide-long 5'-noncoding region. Three different portions of this region were identified as possible sites of interaction with the 3'-end of 18S rRNA that may facilitate efficient rate of translation of the mRNA. Cortisol 21-mesylate was previously shown to exert and anti-glucocorticoid action in rat hepatoma cell culture. When this agent was examined in organ culture of mouse mammary gland, it produced a glucocorticoid-like effect in stimulating milk protein synthesis. The potency of cortisol mesylate was about 1/10 to 1/30th of that of cortisol. These results indicate that the effect of cortisol mesylate can be agonist-like or antagonist-like, depending on its target tissues. The differentiative potential of the lactating mouse mammary epithelial cells were examined in the organ culture system. The data indicate that in the presence of insulin, cortisol and prolactin mammary epithelial cells in culture first lose and then regain the ability of synthesizing milk protein without requiring the formation of new daughter cells.